DEMANDS for a “popular front” to challenge the Tories at the next general election grew louder yesterday as leading figures from four parties met to discuss the idea.

Labour’s Clive Lewis, Green leadership candidate Caroline Lucas, former Lib Dem minister Vince Cable and SNP MP Tommy Shepherd began to hammer out the details for a pact at a meeting of 1,000 activists in London.

The cross-party Compass think tank, which organised the meeting, has long campaigned for an electoral alliance but the prospect of a snap general election later this year — and the attempted coup in the Labour Party — has forced the issue.

Compass chairman Neal Lawson said: “With the right in the ascendancy, a popular front of ideas and organisation is the only way to defend and promote our vision of a good society.”

The Greens took the first steps last week by writing to the leaders of Labour, the Lib Dems and Plaid Cymru to call for a formal meeting to discuss an electoral alliance in England and Wales.

It would mean a change of tack for the Lib Dems, whose leader Tim Farron has sought to capitalise on Labour’s “chicken coup” by saying rebel MPs could defect to his ramshackle outfit.

Green MP Caroline Lucas said last night that any pact was likely to produce “locally driven electoral alliances” in order to defeat Tory candidates in swing seats.

“The overwhelming priority for progressives must be replacing the Tories with a government committed to social justice and environmental protection,” she said.

Although she said her party’s commitment was “contingent on a commitment to fair elections” that would end the need to form alliances to win first-past-the-post elections.

Mr Lewis was said to be committed to starting a dialogue about a pact within Labour but any suggestion that the party does not field candidates in some constituencies will meet resistance among party activists.